A plug cap of the above type is described in Japanese Utility Model Publication Showa 61/61778. FIG. 3 hereof shows such a known plug cap, in which ignition pipe 19 is fitted at one end into a spark plug and receives the plug cap on an external circular face at the other end. The above-mentioned plug cap also incorporates a rain cover and pipe holder 15 fits on the external circular face of ignition pipe 19. Seal 12 is provided between the external circular wall of pipe holder 15 and the internal circular wall adjacent the opening of elongated bore 21 adapted to receive the spark plug. Narrow gap 26 is formed between pipe holder 15 and seal 12. Plug cap 22 is provided with peripheral cover 11 at its external face. This cover engages above and around the opening via ring-shaped rim 17 surrounding the opening. Ignition pipe 19 receives ignition cable 18 which passes through the rain cover. Plug cap 22 thus fits over elongated bore 21, for example, in vehicle engine 20 so that it can be freely mounted or removed.
In this prior art device, proper sealing is ensured by covering bore 21 with peripheral cover 11. Gap 26 between pipe holder 15 and seal 12 provides the resilience required for easy insertion of the plug cap into bore 21, and also prevents any compressive strain from forming in the plug cap during insertion. However, pipe holder 15 and seal 12 are partially separated from each other by gap 26; hence, seal 12 is very thin. Consequently, when plug cap 22 is inserted into bore 21, it tends to curl up at the periphery or otherwise deform, resulting in bore 21 no longer being sealed properly.